I recently bought a Fender Blues Jr. with Billm mods. I wanted a Hammond 1590H footswitch to fit on my pedalboard.
The Blues Jr. footswitch circuit has a 15V signal that is connected to switch the fat switch. Billm suggests that you could use a LED and resistor and then bypass it so that the LED is on when the fat switch is off. However other companies offer a footswitch with an LED that is on when the fatswitch is engaged. I did some testing and I found that, at least with the Blues Jr. III, even with the LED and resistor in the circuit it still switches, so you can just break the circuit to turn it off. I think I calculated that an 800ohm resistor was about right.
Here’s a picture with the LED indicating the fat switch is on: